Frequently the assembly, maintenance, and repairing of structures and systems inside of aircraft wings, for example, make it necessary that access openings are provided in the respective shell structural components. The respective access can be provided by manholes, handholes, or by removable shell sections. Generally, a manhole is cut into a lower wing shell of a wing formed by upper and lower wing shells. For the construction of access openings with a cover, the following construction principles are basically useable.
First, there are so-called no-load carrying manhole covers. Such no-load covers distinguish themselves by the following advantages. It is easy to assemble and disassemble the cover, there is no need for a heavy load carrying cover, the accessibility is good, and there are no bore holes in the load carrying wing skin. However, there is the disadvantage that such a structure is the heaviest of all possible structures of this type. This weight problem is due to the unavoidable fact that additional supporting material in the wing is necessary to lead present forces around the opening. Thus, the cover itself may not be too heavy, but the wing and cover together become heavier.
Second, there are load carrying covers which distinguish themselves by the following advantages. Load carrying covers result in a lighter-in-weight structure compared to structures with no-load carrying covers, because normal- and shear forces can be transmitted totally or partially by the covers. Load carrying covers are not dead weight. Last, but not least, the force flow is less disturbed than in structures with a no-load carrying cover. However, load carrying covers have the following disadvantages. The assembly and disassembly is difficult because of the large number of connecting elements. Force transmission by screw connections has difficulties of its own. In the root zone of a wing two or three lines of screw connections are necessary.
Third, there are load carrying shell elements which are part of a structure in which one or several shell elements, e.g. panels, including the respective stringers are constructed to be dismantable for providing the necessary access. Such shell element construction has the following advantages. Most favorable low weight solutions can be realized. The force flow or transmission is disturbed the least and the respective panels do not constitute dead weight. Besides, in comparison to all other constructions an optimal access is possible.
However, load carrying shell elements have the following disadvantages. Because of the necessary stringer couplings substantial problems occur in the assembly or disassembly. The weakening of a wing when a panel section is removed can possibly require a support of the wing during maintenance work or the like. Difficulties occur at the cross-butt joints of the panels concerning the force transmission. Large normal forces act on panels of this kind, which have to be transmitted.